Retired neurosurgeon and onetime Republican presidential candidate Ben Carson is set to endorse Donald Trump, a rival who once mocked him, on Friday, the billionaire frontrunner said.

Carson himself told Fox News radio on Thursday that "I'm certainly leaning" towards a Trump endorsement, and an aide confirmed that Carson and Trump met recently.

Hours later, Trump said at the Republican presidential debate in Miami that the former candidate, who dropped his bid last week, will endorse him at a press conference Friday in Florida.

Trump said he met Thursday with Carson, who "is endorsing me, by the way, tomorrow morning".

Florida, a main prize in the 2016 nominations race, votes Tuesday, and Trump is ahead there in polling. He is seeking to best Senator Marco Rubio, who sees his home state as a do-or-die contest.

The religiously conservative Carson's outsider campaign soared last year but ultimately lost steam as Carson stumbled over questions about the veracity of aspects of his compelling life story.

MOCKED CARSON

Trump also mercilessly taunted and mocked Carson on the campaign trail, particularly over Carson's claim that he once tried to stab a fellow student but that the boy's belt buckle prevented the knife from breaking the skin.

But Trump later sided with Carson over a dispute in Iowa, when Senator Ted Cruz's campaign wrongly told voters that Carson had dropped out of the race before the state's February 1 caucuses.

Carson said he believed the billionaire real estate mogul will be able to put aside his caustic, controversial rhetoric that has become a hallmark of his public appearances.

"There are two Donald Trumps. There's the Donald Trump that you see on television and who gets out in front of big audiences, and there's the Donald Trump behind the scenes," Carson told Fox.

"They're not the same person. One's very much an entertainer, and one is actually a thinking individual."